Method and devices for improving the longitudinal stability of helicopters

ABSTRACT

The technical province of this invention is that of aeronautical engineering. The invention relates to a method and to devices for improving the longitudinal stability of helicopters, in which an incipient tendency for the nose of the helicopter to pull up is compensated by generating a diving moment responsively to a hinged stabilizer actuated as a result of the inertia of a weight attached thereto. The principal industrial applications of the invention are to be found in the realm of rigid-rotor helicopters.

United States Patent Certain 51 Feb. 20, 1973 [54] METHOD AND DEVICESFOR IMPROVING THE LONGITUDINAL STABILITY OF HELICOPTERS [75] inventor:Bernard Maurlce Certain, Aix-en- Provence, France [73] Assigncc: SocieteNationale lndustrielle Aerospatiale, Paris (Seine), France 221 Filed:Nov.25, 1970 [21] Appl. No.: 92,631

[30] Foreign Application Priority Data Nov. 27, 1969 France ..6940879[52] US. Cl. ..244/l7.l3, 244/l7.19, 244/87 [51] Int. Cl ..B64c 5/10[58] Field of Search..244/17.13, 17.19, 17.11, 17.17, 244/17.15, 87-90[56] References Cited UNITED STATES PATENTS Goland et a1. ..244/17.l3 X

2,630,985 3/1953 Sherry ..244/17.l9 2,941,792 6/1960 Stutz 3,081,0523/1963 Michel ..244/l7.l9

Primary ExaminerTrygve M. Blix Assistant Examiner-Paul E. SaubercrAtlomeyWatcrs, Roditi, Schwartz and Nisscn [57] ABSTRACT The technicalprovince of this invention is that of aeronautical engineering.

The invention relates to a method and to devices for improving thelongitudinal stability of helicopters, in which an incipient tendencyfor the nose of the helicopter to pull up is compensated by generating adiving moment responsively to a hinged stabilizer actuated as a resultof the inertia of a weight attached thereto.

The principal industrial applications of the invention are to be foundin the realm of rigid-rot0r helicopters.

5 Claims, 13 Drawing Figures PATENTED FEB 2 01975 SHEET 10F 4 PATENTEDFEB 2 0 I975 SHEET 30F 4 METHOD AND DEVICES FOR IMPROVING THELONGITUDINAL STABILITY OF HELICOPTERS The technical province of thisinvention is that of aeronautical engineering.

Conventional rotary-wing aircraft with their rotor blades hinged to thehub are affected by longitudinal instability. In forward flight, theresultant of the pulling forces, which is exerted through theaerodynamic center of the rotor, in conjunction with the drag applied tothe fuselage, give rise to an unstable diving moment about the center ofgravity of the machine that increases with increasing flight speed. Thisdiving moment is countered by an opposing moment which, inter alia, maybe generated by a negative-lift horizontal stabilizer positioned at therear end of the fuselage.

A similar phenomenon of instability in the longitudinal attitude is tobe observed in helicopters having flexible rotor blades. At high speedsa sudden increase in this instability may occur as the result of a gustor a change in cyclic pitch. Thus, in level flight, if the machine wereto be left to itself it would tend to pull up, resulting in positiveload factors which could reach high values.

This phenomenon of instability, which is commonplace on rotorcraftequipped with such so called rigid rotors, is attributed to a couplingeffect between the motions of the blades due to .drag and to bladetwist.

Inasmuch as the design of rotors of this kind already raises delicateproblems to solve, modifications to such rotors have not beencontemplated.

It is the object of the present invention to overcome this longitudinalinstability of helicopters and to accordingly provide a stabilizationmethod whereby any tendency to diverge from the longitudinal attitude iscompensated by engendering a reverse moment on the machine responsivelyto an inertiaoperated stabilizer.

Thus an incipient pulling-up moment instantly gives rise to a divingmoment.

In a preferred embodiment of the method of this invention, said divingmoment is generated by the inertia of a pendular weight, the action ofwhich is thus proportional to the increase in the load factor butindependent of the aerodynamic forces.

The invention further relates to devices for performing such method thatinclude in particular a horizontal stabilizer positioned at the rear endof a helicopter fuselage, which stabilizer is pivotally mounted about ahorizontal shaft and is fast with a weight the inertia of which deflectssaid stabilizer in response to incipient longitudinal divergence fromthe flight path, such deflection being appropriate to counter saiddivergence.

In a preferred embodiment, said weight is supported on the end of alever arm substantially parallel to the chord of the stabilizer airfoil,said lever being directly or indirectly connected to a return springwhich, by acting with respect to the instantaneous fulcrum of saidstabilizer together with the aerodynamic forces exerted against thelatter, balances the moment generated by said weight, the rating of thespring being determined by the maximum stabilizer deflection setting atwhich stability is desired.

In accordance with a first alternative embodiment in which a horizontalstabilizer of this kind is mounted on top of a vertical stabilizer, thelever arm and its pivotal elements are arranged to form a support forsaid horizontal stabilizer on an intermediate part pivotally mounted ontop of the vertical stabilizer, said intermediate part being rigid withmeans for setting it angularly with respect to the upper edge of thevertical stabilizer, which angular setting means permit adjustment ofthe balancing position of the horizontal stabilizer, the latter beingset to provide negative lift in normal flight.

The horizontal stabilizer support is formed by an inverted channelsection which caps the intermediate part, which part is formed in turnby braced flanges.

The liaison between the horizontal stabilizer support and said flangesis provided by two divergent links which are mounted, on the one hand,on two upper shafts fast with the upper parts of the inner flanges and,on the other, on two subjacent shafts which are more widely spaced thansaid upper shafts and are rigid with said support. This balancingpendular articulation system permits stable pivotal motion of thehorizontal stabilizer, which is normally at negative incidence, aboutthe first-mentioned two shafts which are connected to the verticalstabilizer, the other two articulation shafts rigid with the horizontalstabilizer support being preferably included in a plane parallel to theplane containing the chord of said horizontal stabilizer airfoil.

One of the flanges is fast with a balancing-positionsetting bellcrankwhich is attached to a tie rod formed by a bracket supported on thevertical stabilizer.

In order to obtain, from the aerodynamic standpoint, sufficient dampingof the horizontal stabilizer motion subsequent to actuation thereof, thecorresponding articulation is associated to a damper possibly positionedbetween the two flanges and having its body portion fast with onethereof, the other flange being equipped with an arrangement ofadjustable stops positioned in registry with contacting surfaces formedon the side of the horizontal stabilizer support.

With such an arrangement, the instantaneous fulcrum about which thehorizontal stabilizer rotates and which is at all times located at theintersection of two planes defined respectively by the shafts rigid withthe flanges and by the shafts rigid with the supports, is caused toshift rearwardly along a locus which is a circular arc as the motionsrequired to achieve the desired stabilization take place.

The position at which the instantaneous fulcrum coincides with theaerodynamic center is the position at which the system is in a state ofequilibrium.

In an arrangement as hereinbefore described, when the load factorincreases, the moment developed by said weight increases and tends tocause the horizontal stabilizer which the weight is rigid assumepositivegoing incidence, whereby a modified lift value is produced thatgenerates a diving moment for the helicopter about its center ofgravity, which diving moment counters the pull-up moment due to thelongitudinal divergence and thereby ensures the required stabilizationof the helicopter about its pitch axis.

In a second alternative embodiment likewise designed to improvelongitudinal stability, the rear end and, mounted thereon, a weightsecured to the rear ofa formed section rigid with the root of eachstabilizer element. A spring connected between the forward part of thiselement and an associated end of a horizontal rocker arm fast with thetail boom balances the moment generated mainly by said weight.

Pivotal motion of the said formed sections responsively to the inertiaof the associated weights, in order to cause the horizontal stabilizerto react at the onset of a pitch-up divergence by the machine, takesplace about the axis of a tube carrying the two horizontal stabilizerelements and is limited by two stops.

With such an arrangement there is likewise provided a damper the bodyportion of which may be disposed within said formed section.

Possible industrial applications of the invention are to be found in thefield of helicopters, and more particularly helicopters having rotorswith flexible blades devoid of hinges.

The description which follows of a number of nonlimitative exemplaryembodiments of devices for improving the longitudinal stability ofhelicopters, given with reference to the accompanying drawings, willgive a clear understanding of how the invention can be cartied intopractice.

In the drawings:

FIG. 1 schematically illustrates the location of the subject device ofthe invention on top of a vertical stabilizer on a helicopter FIG. 2shows in side elevation a horizontal stabilizer equipped with adeflecting system according to the invention FIG. 3 is a section takenthrough the line III--III of FIG. 2

FIG. 4 is a section taken along the line IV-IV of FIG. 2

FIG. 5 is a section taken along the line V-V of FIG.

FIG. 6 is a section taken along the dash line VIVI of FIG. 2

FIG. 7 is a section taken through the line VII-VII of FIGS. 2 and 8 FIG.8 is a side elevation view which is the reverse of that of FIG. 2;

FIG. 9 is a section taken through the line IX-IX of FIG. 8

FIG. 10 is a fragmental showing, in corresponding fashion to FIG. 1, ofan alternative arrangement for a stabilizing device according to thisinvention FIG. 11 shows in side elevation on an enlarged scale a controlarrangement applicable to the embodiment of FIG. 10

FIG. 12 is a plan view corresponding to FIG. 11, with partial cutawayand FIG. I3 is a section taken through the line XIII-XIII of FIG. 11.

As shown in FIG. 1, the vertical stabilizer D of a helicopter l-I havinga rotor R carries at its top a horizontal stabilizer E for stabilizingthe longitudinal attitude of the helicopter, more particularly when saidattitude tends to be disturbed by gusts or sudden changes in the cyclicpitch of the rotor blades.

Reference is next had to FIG. 2, in which the top 1 of the helicoptersvertical stabilizer D receives an element 2 forming a horizontalstabilizer E. This arrangement is effected by means of a support 4formed by an inverted channel section having its web applied against thelower surface 3 of element 2 and secured thereto by screws 5 and 6 andblocks 7 and 8 permitting precise mutual positioning of support 4 andairfoil 2. The web and flanges of support 4 are suitably cut to shape.and there is secured to the rear end of support web 9, by means of bolts10 and 11, a weight 12 made of heavy alloy and of size appropriatelyadjusted'thickness-wise.

The flanges l3 and 14 of support 4 are form'edwith correspondingopenings 15 and 16 shaped substantially as trapeziums having theirlonger bases respectively forming the upper edges of said openings.

Attached to the upper edge 1 of the vertical stabilizer are two verticalflanges l7 and 18 which jointly form an intermediate part and whichembody circular openings 19 and 20 respectively, in registry with eachother. The spacing between flanges 17 and I8 is maintained by spacermeans, one of which is secured by a bolt and nut 24.

In respect of these two flanges, such attachment is effected, on the onehand, upon a shaft 21 extending through one of said spacer means andrestrained in lugs 22 of said flanges and in the sides of a clevis 23fixed to the top of stabilizer D, in the forward region thereof. Theattachment system is completed, on the other hand, by having flange l8bear at its rear a bellcrankforming extension 25 which is bolted at 26and 27 to rear lugs on flange 18.

The tapering end of bellcrank 25 is secured by a bolt and nut 28 to aslotted portion 29 of a bracket 30 secured by bolts and nuts 31 to aclevis 32 likewise carried on the crest line 1 of vertical stabilizer D.Bolt 28 is movable along slot 33 of bracket 30 in order to enable theequilibrium position of horizontal stabilizer E to be set, the need forwhich setting will become apparent hereinafter.

Adjacent the rear end of its crest line 1, vertical stabilizer D carriesa bracket 35 from which extends a threaded rod 34 associated to a nutand locknut, said rod terminating in an eye 36 into which is hooked theend of a spring 37.

The rating of spring 37 is determined by the maximum angular setting ofthe horizontal stabilizer for which stability is desired, and the otherend of spring 37 engages in a groove 38 formed in the middle region of aspacer pin 39 extending through the flanges 13 and 14 of horizontalstabilizer support 4 (FIGS. 6 and 9). The lower part 40 of bracket 30(FIG. 5) forms a yoke between the arms of which spring 37 freelyextends, each yoke arm being provided with a bolt and nut 31.

The hinge coupling between stabilizer E and stabilizer D, which is anindirect coupling, is accomplished through the agency of theintermediate member formed by flanges 17 and 18, the angular positionsof which are adjustable about shaft 21 and which are associated to twolinks 41 and 42 (see FIGS. 2, 3 and 4). Links 41 and 42 are hingedlyconnected to the forward upper ends respectively of flanges 17 and 18through the medium of spacer pins 43 and 44.

Links 41 and 42 splay out downwardly from pins 43 and 44 and areinterconnected in corresponding pairs by pins 45 and 46 supported inbearings 47 and 48, the latter being fast with lower portions of theside plates or flanges l3 and 14 of support 4 by means of securing lugs49. Pivotal motion of links 41, 42 about upper pins 43 and 44 isfacilitated by needle-bearings 50 and bores 51 in the link ends, saidneedle-bearings being spaced as required by central cylindricaldistance-pieces 52 and flanged washers 53 and 54 pivotal motion of saidlinks about the lower pins 45 and 46 is likewise facilitated byidentical needle-bearings driven into bores in the other ends 55 of saidlinks and maintained in spaced relationship by a central distance-piece52 and two flanged bushes 56 and 57.

A peg 58 (FIG. 3) equipped with a flag marker may be inserted throughaligned holes 59 in flanges l3 and 14 of support 4 and in the lugs 49 ofbearings 47 and 48, and thereafter in the link 42, these last holeslying in the plane containing pins 44 and 46. Peg 58, shown in theengaged position in FIG. 3, is designed to restrain the stabilizersupport 4 in relation to flanges 17 and 18 when the resting position ofequilibrium of the horizontal stabilizer has been set subsequent tosuitable adjustment of the mass of material required to form the weightreferred to precedingly.

Fitting and removal of pins 43 and 44 with respect to flanges 17 and 18are possible by the access provided through openings and 16 of unequalsize formed in the sides 13 and 14 of support 4 and through a marginalopening 16 (FIG. 1) associated-to openings 60a in the sides 13 and 14respectively for permitting engagement and disengagement of pin 24. Thecutaway formed in the lower portion of sides 13 and 14 likewise providesaccess to pins 45 and 46 and to pin 21.

In order to provide damping of the motion of horizontal stabilizer Eduring variations in the load factor, a damper (not shown) is positionedbetween the two flanges 17 and 18, the body portion of the damper beingscrewed through the medium of a base to flange 18 (shown in dash linesin FIG. 2). The facing opening 19 in flange 17 provides passage for thehinge-pin of such pivotal damper, which hinge-pin is made fast with acrank to which is hingedly connected a link having its other endpivotally connected to an appropriately configured end of one of pins 45or 46. A linkage system of this kind is shown in dot-dash lines in FIG.2.

Associated to said damper is a system of lengthwiseadjustable stops 61and 62 (FIG. 8) screwed into and restrained by nuts and locknuts in abracket 63 fast with flange 18, which bracket and stops project into thecutout 16 above the lower edge thereof, which edge bears a bracket 64carried internally on the facing flange 14 of support 4. That surface ofbracket 64 which faces the tips of stops 61 and 62 bears liningscooperating with said tips.

The theory of operation of the system described hereinabove is asfollows:

While the helicopter is on the ground, peg 58 is removed after settingthe equilibrium position of horizontal stabilizer E that is required instable flight (by setting a degree of negative lift on said stabilizerto balance the diving moment which is generated by the component of pullexerted at the aerodynamic center of the rotor and by the drag exertedagainst the fuselage) having regard for the reciprocal actions of weight12 and spring 37.

Since in stable flight it is required to have a zero angular setting ofthe horizontal stabilizer, the sum of the moments acting about theinstantaneous fulcrum of said stabilizer must be zero, whereby the needfor the action of spring 37 to counter the moment generated by weight12. Horizontal stabilizer E consequently remains motionless.

In response to a gust or an abrupt change in the cyclic pitch at highforward speeds, the helicopter experiences incipient pitch-up, therebycausing an increase in the load factor. This increase in turn causes themoment generated by weight 12 to increase and to thereby causehorizontal stabilizer E to assume positive-going incidence. Two momentsoppose this motion, one of which is due to the spring 37 and the otherto the aerodynamic forces acting downwardly on stabilizer E.

The equilibration of the moment due to weight 12, to spring 37 and tosaid aerodynamic forces determines a different angular setting ofhorizontal stabilizer E and hence a different lift value therefor,whereby a force is caused to act upwardly and to engender a divingmoment about the center of gravity of the helicopter. This diving momentopposes the pitchup moment which accompanies the incipient longitudinaldivergence referred to precedingly, whereby the required stabilizationof the longitudinal attitude of the helicopter is achievedautomatically.

In the alternative embodiment depicted in FIGS. 10 to 13, thelongitudinal stability improving device is mounted on the helicoptertail boom P in the form of two horizontal half-stabilizers E, projectingfrom either side of boom P. In FIGS. 11 and 12, only one horizontalstabilizer element 66 is shown in addition to the tail boom skin 65.

Secured to the root 67 of element 66 is a tapering channel-section 68having its open side facing skin 65. Attached to the rear end of channelsection 68 is a weight 69 made of heavy alloy. Forward thereof isdisposed, on the web of section 68, a damper 70 the axle of whichcarries a crank 72 pivotally connected to a link 73 which is in turnpivotally connected to a hinge-pin 71 fixed to skin 65.

The forward end of section 68 is provided with means for attaching thelooped end of a spring 69a, through the agency of retention meansthreaded over a bolt 75. The other end of spring 69a (FIG. 13) isattached to a metal strip welded to a threaded rod 76 which is securedby a nut and locknut to one end of a substantially horizontal transversearm 77 which is fulcrumed by means of a hinge-pin in a clevis 78 rigidwith the undersurface of tail boom skin 65. Arm 77 accordingly forms arocker arm interconnecting the two stabilizer elements.

Additionally fixed to the sides of the tail boom are T- sections 79 thewebs 80 of which are cut to match the shape of skin 65 and welded overtheir entire lengths thereto, and the flanges 81 of which have boltedinto slots therein two brackets 82 and 83 faced with elastic linings 84against which the sides of channel section 68 bear alternately.

The positioning of these abutment brackets and the spacing impartedthereto are determined, on the one hand, according to the restingposition which it is desired for stabilizer element 66 to assume, and onthe other according to the maximum deflection of which said element isto be capable in the event of longitudinal divergence of the helicopterdue to pitch-up caused by a gust and/or a sudden variation in cyclicpitch.

The two stabilizer elements 66 are fast with a common tubular spar 85which is carried in antiafriction bearings 86 supported in associatedhousings in the tail boom and by means of which said elements arecapable of pivotal motion.

As stated precedingly, the two horizontal stabilizer halves areidentically devised and are positioned symmetrically with respect to thehelicopter fore-aft axis.

In the event of incipient pitch-up by the helicopter, the two stabilizerelements react by pivoting about the spar axis in response to anyincrease in the load factor causing the weights 69 to react. Suchreaction determines a modified angle of incidence of the horizontalstabilizer and a consequently modified lift value thereof. With thearrangements described hereinabove, this results in the generation of adiving moment which stabilizes the helicopter about its center ofgravity.

It goes without saying that changes and substitutions may be made to thespecific forms of embodiment hereinbefore described without departingfrom the scope of the invention. By way of example, the stabilizerelements could be provided with aerodynamic control surfaces known astabs, capable of being deflected with respect to the chord line of theairfoil section of such stabilizer in order to cancel the effect of anaerodynamic pitching-up moment about the instantaneous fulcrum of saidstabilizer element.

What I claim is:

1. In a device for improving the longitudinal stability of a helicopterhaving a rotor provided with flexible blades deprived of articulationsfor flapping and leadlag oscillations, in which at least one horizontalstabilizer element is hingedly mounted on said helicopter on a firstaxis parallel to pitch axis of said helicopter and is rigid with aweight, said weight being situated at one extremity of a lever armnearly parallel to the airfoil chord of said stabilizer element saidlever being attached to a return spring, an intermediate member beinginterposed between helicopter body and hinge of said stabilizer element,said intermediate member being itself hingedly mounted on a second axisparallel to said first axis, the improvement according to which saidlever is forming a horizontal stabilizer Supporting element straddlingsaid intermediate member constituted of cross-braced flanges, supportedby said second axis on the first hand and attached, on the other hand,by an interconnecting arm to a slotted bracket, said second axis andsaid bracket being affixed to crest line of a vertical stabilizer of afuselage of said helicopter, said horizontal stabilizer over-lying saidcrest line.

2. In a device according to claim 1, a flange supporting a damper bodyand adjustable stop means situated in front of contact surfaces formedin said lever, said damper having a movable part mechanically connectedto said lever.

3. In a device for improving longitudinal stability of a helicopterhaving a rotor provided with flexible blades deprived of articulationsfor flapping and lead-lag oscillations, in which at least one horizontalstabilizer element is hingedly mounted on said helicopter on a firstaxis, parallel to pitch axis of said helicopter, said horizontalstabilizer element being fast with a weight situated at one end of alever arm nearly parallel to airfoil chord of said stabilizer element,said lever arm being attached to a return spring, the improvementaccording to which said stabilizer element is supported by a sparcarried as an axle in a bearing housing inserted in a helicopter tailboom, said stabilizer element having a root portion fast with a leversupporting at its rear end a high-density weight and attached at itsforward end to a spring itself attached at one extremity of an armhingedly fixed to said tail-boom, said forward end being rockablebetween two adjustable abutment means rigid with said tail boom.

4. In a device as claimed in claim 3, an arrangement, symmetrical withrespect to the longitudinal centerplane of said tail boom, of empennageelements, of levers, of weights, of springs and of said arm, the latterbeing supported for rocking motion about an axis lying in saidlongitudinal plane.

5. In a device as claimed in claim 3, damper means having its stationaryportion supported by said lever and its movable portion attached topivot means rigid with said tail boom.

1. In a device for improving the longitudinal stability of a helicopterhaving a rotor provided with flexible blades deprived of articulationsfor flapping and lead-lag oscillations, in which at least one horizontalstabilizer element is hingedly mounted on said helicopter on a firstaxis parallel to pitch axis of said helicopter and is rigid with aweight, said weight being situated at one extremity of a lever armnearly parallel to the airfoil chord of said stabilizer element saidlever being attached to a return spring, an intermediate member beinginterposed between helicopter body and hinge of said stabilizer element,said intermediate member being itself hingedly mounted on a second axisparallel to said first axis, the improvement according to which saidlever is forming a horizontal stabilizer supporting element straddlingsaid intermediate member constituted of crossbraced flanges, supportedby said second axis on the first hand and attached, on the other hand,by an interconnecting arm to a slotted bracket, said second axis andsaid bracket being affixed to crest line of a vertical stabilizer of afuselage of said helicopter, said horizontal stabilizer over-lying saidcrest line.
 1. In a device for improving the longitudinal stability of ahelicopter having a rotor provided with flexible blades deprived ofarticulations for flapping and lead-lag oscillations, in which at leastone horizontal stabilizer element is hingedly mounted on said helicopteron a first axis parallel to pitch axis of said helicopter and is rigidwith a weight, said weight being situated at one extremity of a leverarm nearly parallel to the airfoil chord of said stabilizer element saidlever being attached to a return spring, an intermediate member beinginterposed between helicopter body and hinge of said stabilizer element,said intermediate member being itself hingedly mounted on a second axisparallel to said first axis, the improvement according to which saidlever is forming a horizontal stabilizer supporting element straddlingsaid intermediate member constituted of cross-braced flanges, supportedby said second axis on the first hand and attached, on the other hand,by an interconnecting arm to a slotted bracket, said second axis andsaid bracket being affixed to crest line of a vertical stabilizer of afuselage of said helicopter, said horizontal stabilizer over-lying saidcrest line.
 2. In a device according to claim 1, a flange supporting adamper body and adjustable stop means situated in front of contactsurfaces formed in said lever, said damper having a movable partmechanically connected to said lever.
 3. In a device for improvinglongitudinal stability of a helicopter having a rotor provided withflexible blades deprived of articulations for flapping and lead-lagoscillations, in which at least one horizontal stabilizer element ishingedly mounted on said helicoPter on a first axis, parallel to pitchaxis of said helicopter, said horizontal stabilizer element being fastwith a weight situated at one end of a lever arm nearly parallel toairfoil chord of said stabilizer element, said lever arm being attachedto a return spring, the improvement according to which said stabilizerelement is supported by a spar carried as an axle in a bearing housinginserted in a helicopter tail boom, said stabilizer element having aroot portion fast with a lever supporting at its rear end a high-densityweight and attached at its forward end to a spring itself attached atone extremity of an arm hingedly fixed to said tail-boom, said forwardend being rockable between two adjustable abutment means rigid with saidtail boom.
 4. In a device as claimed in claim 3, an arrangement,symmetrical with respect to the longitudinal centerplane of said tailboom, of empennage elements, of levers, of weights, of springs and ofsaid arm, the latter being supported for rocking motion about an axislying in said longitudinal plane.